The magnetic recording of information is a well-known and widely used method. Depending on the application, the information is recorded on tape or rigid or flexible cards or discs, and in analogue or digital form. Analogue recording is used for sound reproduction, for example, and digital recording in data processing.
The recording methods currently in use enable the recording media to be re-used many times over, each new recording erasing and replacing the previous one.
This fundamental characteristic of current methods is sometimes an advantage, but it can also be a disadvantage. If applied to certain types of document, such as credit cards or passports, for example, these methods do not result in sufficient resistance to accidental or intentional interference with the recorded information.
A method of making permanent magnetic recordings is known in which a support is provided with layer of anisotropic magnetisable particles which are uniformly distributed in a flexible binder and mixed with a solvent. All the particles are then oriented in a predetermined direction by means of a magnetic field and then the particles in certain zones corresponding to the configuration of the message to be recorded are selectively oriented in a perpendicular direction, by means of small discrete magnets for example. The layer is then dried by heating to evaporate the solvant and to fix the particles in position. Such a method is described in U.S. Pat. No. 3,873,975.
However, in this method of selective orientation, unfixed and differently oriented particles coexist adjacent each other in the same layer before drying. These particles exert forces on each other tending to rotate the particles before they are fixed, thereby causing indistinct boundaries between zones of different orientation.